1. Field of the Invention
This invention relates to the field of switching power supplies, and more particularly, to a flyback regulator for providing a low voltage power supply and a high voltage power supply.
2. Description of the Related Art
Audio, graphics, and data processing requirements in portable devices such as laptop computers are constantly increasing. The increased processing capability is accompanied by increasing power dissipation and heat. There is a corresponding requirement for portable power supplies that are as energy and space efficient as possible. Temperature management is also a high priority, as elevated temperatures may adversely affect a device's reliability. To meet power requirements, the trend in the industry is to use a greater number of power supplies that provide lower voltages and increased current.
Portable electronic devices are typically capable of operating with regulated DC power supply from a portable, rechargeable battery pack, or from an AC power supply using an AC to DC adapter having one end that plugs into an electrical socket and another end that plugs into the device. The adapter supplies power to operate the device as well as to recharge the battery pack. AC adapter power conversion and battery recharging is accomplished most efficiently at high output voltage. To improve battery charging efficiency, it is therefore desirable to provide a high voltage signal to charge batteries.
Many portable electronic devices utilize Lithium icon (Li-ion) batteries which are capable of providing both high voltage and excellent capacity, resulting in relatively high energy density. The internal impedance of Li-ion batteries is very high, however, and the batteries are therefore more efficiently utilized in circuits; that require high voltage and low current. This characteristic runs contrary to the industry trend to use lower voltages and increased current.
Converting DC voltage supplied by the battery or AC adapter to regulated DC voltage supplied to the device is accomplished most efficiently in situations where there is a low input to output voltage differential. This characteristic also runs contrary to the industry trend to utilize lower operating voltage, however, since low operating voltages increase the differential between the input and the output voltage. The differential voltage depends on the voltage delivered by the battery or the AC adapter and is typically greatest when utilizing power supplied by the AC adapter. To improve conversion efficiency, it is therefore desirable to generate a low voltage input signal that may be converted to regulated DC voltage supplied to the device.
Re-chargers for portable batteries utilize switching regulators to regulate DC power input to the battery pack. Switching regulators are typically classified into different configurations or "topologies." One such topology is the inverting or "flyback" regulator where a switch determines whether the voltage applied to an inductor is the input voltage, V.sub.dc, or zero. In this manner, the output voltage is a function of the average voltage applied to the inductor. The regulator controls; the turning ON and turning OFF of the switch in order to regulate the flow of power to the load. The switching regulator employs inductive energy storage elements to convert the switched current pulses into a steady load current. Power in a switching regulator is thus transmitted across the switch in discrete current pulses.
In order to generate a stream of current pulses, switching regulators typically include control circuitry to turn the switch on and off. The switch duty cycle, which controls the flow of power to the load, can be varied by a variety of methods. For example, the duty cycle can be varied by either (1) fixing the pulse stream frequency and varying the ON or OFF time of each pulse, or (2) fixing the ON or OFF time of each pulse and varying the pulse stream frequency. Which ever method is used to control the duty cycle, the switch in switching regulators is, either OFF, where no power is dissipated by the switch, or ON in a low impedance state, where a small amount of power is dissipated by the switch. This generally results in fairly efficient operation with regard to the average amount of power dissipated.
In view of the foregoing, it is desirable to provide an AC adapter utilizing a switching regulator that is capable of supplying a plurality of different voltages to efficiently meet the device's low voltage and high voltage requirements.